The following method is generally used when obtaining the systolic blood pressure by a blood pressure measuring method using a compression air bladder. That is, the blood flow in an artery is stopped by raising the internal pressure of the compression air bladder such that the internal pressure is equal to or higher than the systolic blood pressure as the peak pressure of the intra-arterial pressure. After that, the internal pressure of the compression air bladder is gradually lowered, and a phenomenon in which the blood reflows when the systolic blood pressure matches the pressure of the compression air bladder is detected.
A blood pressure measuring apparatus using the Korotkoff method (auditory method) presently widely adopted as a method of detecting this blood reflow, the blood flow is stopped by making the internal pressure of the compression air bladder equal to or higher than the systolic blood pressure, and the pressure of the compression air bladder is gradually lowered. Korotkoff sounds (K sounds) generated at the timing at which the blood flow once stopped reflows are detected in a peripheral portion on the downstream side of the compression air bladder or a cuff. The internal pressure of the compression air bladder at the same timing is obtained as a systolic blood pressure value.
On the other hand, in an oscillometric-type blood pressure measuring apparatus, the internal pressure of the compression air bladder is once raised to a high pressure equal to or higher than the systolic blood pressure as in the Korotkoff method (auscultatory method) described above, but the occurrence of a blood flow to the cuff downstream side cannot clearly be detected. Instead, therefore, the vibration of the compression air bladder based on the change in volume of the artery, which occurs when the internal pressure of the air bladder is gradually lowered, is detected. The blood pressure is determined by the profile of the change in amplitude of this vibration.
The above-mentioned, oscillometric-type blood pressure measuring apparatus is designed to obtain a blood pressure value from the vibration of the air bladder by the profile of the change in internal pressure of the air bladder. This obviates the need for a microphone or stethoscope for detecting the Korotkoff sounds, which is essential in the Korotkoff method. Accordingly, the oscillometric-type blood pressure measuring apparatus has the advantages that the number of parts can be made smaller than that of the Korotkoff method, and the manufacturing cost can be decreased because an electronic circuit for detecting the K sounds (Korotkoff sounds) can be omitted.
In addition, the Korotkoff-type blood pressure measuring apparatus is readily influenced by noise caused by the frictional sound of cuff cloth or a cuff tube generated during measurement, and external noise such as the noise of an air-conditioning apparatus or human voices. Even when using the frequency discrimination method, the Korotkoff-type blood pressure measuring apparatus is still weak against these noises because the frequency components of the noises are close to those of the Korotkoff sounds.
On the other hand, the frequency components of the pressure fluctuation used in the oscillometric-type blood pressure measuring apparatus are lower than those of the Korotkoff sounds and largely dissociated from the frequencies of the external noise. Therefore, the oscillometric-type blood pressure measuring apparatus is not easily influenced by noise.
Unfortunately, even the oscillometric method has a drawback: the oscillometric-type blood pressure measuring apparatus has a problem pertaining to the detection of the systolic blood pressure, which is caused by the blood vessel pressing characteristic of the compression air bladder or cuff. When pressing the artery of the upper arm as a measurement portion by winding the built-in compression air bladder of the cuff around the upper arm and inflating the compression air bladder, the force of pressing the upper arm generated when the compression air bladder is inflated reflects the internal pressure of the air bladder in a central portion in the widthwise direction (the longitudinal direction of the upper arm) of the air bladder. However, this pressing force reflecting the internal pressure of the air bladder cannot be obtained in the two end portions spaced apart from the central portion and extending along the longitudinal direction of the upper arm. That is, the compression air bladder has the pressing characteristic called a cuff-edge effect that the pressing force gradually reduces from the central portion in the widthwise direction of the air bladder toward the two end portions of the air bladder.
Due to this pressing characteristic, when the cuff internal pressure (the internal pressure of the air bladder) is made equal to or higher than the systolic blood pressure to occlude the blood flow in a measurement portion and then gradually reduced, the blood flow is occluded in only the central portion of the compression air bladder or cuff at the timing at which the cuff internal pressure is slightly higher than the systolic blood pressure. Consequently, the blood flows into the space between the upstream portion and central portion of the cuff in synchronism with the heart beat, and the internal volume of the cuff changes. At the timing at which the cuff internal pressure is higher than the systolic blood pressure, therefore, the change in pulse wave (the increase in amplitude) caused by the change in internal volume of the cuff is detected. This makes it impossible to detect the timing at which the cuff internal pressure becomes lower than the systolic blood pressure (the change in internal volume of the cuff that is an index for the detection of the blood flow occurring downstream of the cuff). This poses the problem that the systolic blood pressure cannot accurately be measured.
Accordingly, the double-cuff method has been proposed to solve the above-mentioned problem in the detection of the blood reflow phenomenon.
This double-cuff method uses a compression air bladder for pressing the blood vessel, and a pulse wave detection air bladder that is separated from the compression function and detects only a pulse wave generated by blood reflow slightly downstream of a central portion below the compression air bladder, thereby reducing the above-mentioned influence of a pulse wave, which is the problem of the oscillometric method, based on the volume change on the upstream side of the compression air bladder when measuring the systolic blood pressure. This makes it possible to detect, at a high S/N ratio, the volume change on the downstream side of the compression air bladder as an index for the determination of the systolic blood pressure.
Unfortunately, at the systolic blood pressure detection timing at which the cuff pressure of the compression air bladder is almost equal to the systolic blood pressure, a blood flow entering the upstream side below the compression air bladder enters the vicinity of the central portion of the compression air bladder, that is, comes very close to the pulse wave detection air bladder. Vibrations caused by this entry are partially transmitted to the pulse wave detection air bladder via a living body. Also, since the pulse wave detection air bladder is formed below the compression air bladder, cuff vibrations based on the volume change of the compression air bladder, which is detected by the compression air bladder, are partially transmitted to the pulse wave detection air bladder. This causes a phenomenon in which the S/N ratio of the systolic blood pressure measurement decreases.
Accordingly, the following proposals have been made in order to prevent the pulse wave detection air bladder from detecting the pulse wave generated by the blood flow entering the upstream side, that is, to prevent the blood flow entering from the cuff upstream side, when the blood vessel is closed with pressure by the compression air bladder, from approaching the pulse wave detection air bladder, by broadening the range within which the blood vessel is closed with pressure. That is, a backing for improving the pressing performance of the pulse wave detection air bladder is installed, a damper for damping the pulse wave transmitted from the compression air bladder is installed between the pulse wave detection air bladder and compression air bladder, and a damper for damping the pulse wave on the upstream side below the compression air bladder is installed (Japanese Patent No. 3667326).
These proposals can increase the pressing force of the pulse wave detection air bladder. However, there are large variations in distance at which the position where the pulse wave enters from the upstream portion of the cuff at the end of the air bladder is spaced apart from the pulse wave detection air bladder. Also, if the distance is short, the pulse wave detection air bladder may detect the upstream pulse wave. Furthermore, the damping characteristic concerning the frequency of a member to be used is limited: it is possible to damp relatively high frequency components of the pulse wave, but it is impossible to sufficiently damp low frequency components. This sometimes makes it impossible to detect the systolic blood pressure at a high S/N ratio.
To solve this problem, it is possible to improve the performance of the damper for damping the pulse wave on the upstream side below the compression air bladder. However, if a cover-like rigid material is used as the damper in order to improve the damping performance, the damper itself inhibits the compression air bladder from pressing an arm especially when the diameter of the arm is small. This produces dissociation between the internal pressure of the compression air bladder and the actual pressure pressing the upper arm. Consequently, there is no means for obtaining the pressure pressing the upper arm at the blood reflow timing. Also, a generally used compressive damping material such as foamed urethane has the drawback that the degree of compression of the damping material changes in accordance with the pressure of the compression air bladder and this changes the damping characteristic. Over the whole pressure range, therefore, the settings of the damper required to fully utilize the damping performance change each time in accordance the shape of an arm and the way the compression air bladder is wound. This makes it difficult for the conventionally proposed methods to prevent a damper having a satisfactory damping performance from inhibiting the compression air bladder from pressing the arm of any person.
Furthermore, as a technique that detects, with high sensitivity, the blood flowing toward the peripheral side of a cuff when the cuff pressure becomes lower than the systolic blood pressure in an oscillometric-type sphygmomanometer, an arrangement has been proposed in which a pulse wave detection air bladder is formed downstream of a cuff so as to increase the sensitivity of detection of a pulse wave on the cuff peripheral side and detect a pulse wave on the cuff downstream side, and a compression air bladder for pressing a whole blood pressure measurement portion is connected to the pulse wave detection air bladder (Japanese Patent Laid-Open No. 63-150051).
Unfortunately, each arrangement described above has a connecting portion for connecting the blood blocking bladder and air bladder. Accordingly, the upstream-side pulse wave is detected as it is superposed on the downstream-side pulse wave. This makes it impossible to increase the S/N ratio for the detection of the systolic blood pressure.